1. Field of the Invention
The present invention generally relates to imaging systems and more particularly relates to an adaptive timing control of the light integration process of one-dimensional CMOS image sensors used in the imaging systems.
2. Description of the Related Art
There are many applications that need an imaging system to convert an object to an electronic format that can be subsequently analyzed, printed, distributed and archived. The electronic format is generally a black-and-white or color pixel image of the object. A typical example of the imaging system is a scanner and the object is generally a picture or a sheet of paper from an article. Through the scanner, an electronic or digital image of the picture or paper is generated and may be used to design World Wide Web pages.
An imaging system includes a sensing module that converts an object optically into an electronic image. Key components in the sensing module include an illumination source, an optical system and an image sensor. The illumination source provides illumination to the object being imaged. The optical system is used to collect and focus the incident light from the object onto the image sensor. The image sensor comprising a large number of photodetectors produces proportional electronic signals in response to the incident light. To reproduce colors of an object, it is known that at least three primary colors, typically red, green and blue, are needed. Analogously, colored lights such as red, green and blue lights are used in the illumination source of the sensing module. The three colored lights are turned on independently and successively so that the image sensor generates three intensity images, each with reference to one of the three colored lights.
The sensitivity (spectral response) of the image sensor to the colored lights, however, is not uniform. For example, a Complementary Metal-Oxide Semiconductor (CMOS) image sensor has a low sensitivity to the blue light but is highly sensitive to the red light. In addition, the luminance intensity of the green light is much stronger than the other two colored lights. Hence equal illumination parameters applied to all three colored lights have rarely considered because that would otherwise result in color biased images.
There have been many efforts to control the illumination parameters. A typical approach is to designate a set of different illumination timing parameters. For example, the illumination time for the blue light is made longer than that for the green light so that the image sensor gets exposed under the blue illumination longer than under the green illumination to produce equally strong intensity signals. Another typical approach is to have a set of different voltages (powers) applied to the three colored lights. For example, a weaker voltage is applied to the green light so as to produce a light to which the image sensor would have the same sensitivity as to the blue light. Besides using the different illumination parameters, some imaging systems further incorporate a compensation circuit or process to neutralize the resultant images due to the uneven sensitivity of the image sensor.
U.S. Pat. No. 5,729,361 to Suggs and Moayer discloses a combination of first providing a coarse adjustment by varying both the power provided to the lights (emitters) and the duration of the emitters/detector exposure time, and then providing a fine adjustment by storing a correction value for each of the photodetector elements. The method uses essentially a set of well controlled different illumination parameters to compensate the uneven sensitivity of an image sensor.
Having a set of controlled different illumination parameters typically demands a supporting circuitry to produce a number of illumination control signals with different characteristics, each controlling one of the lights. Besides the supporting circuitry could be complicated depending on the lights and other control factors, the set of controlled different illumination parameters often requires the image sensor to work precisely with the lights, which can be difficult under a shared bus environment. The bus, such as a PCI bus in a host computer, is a shared resource (bus) which many components and peripheral devices are coupled to for communications with the host computer. These components and peripheral devices then must arbitrate for the mastership of the bus, which inevitably introduces latency associated with the image data being transmitted over the bus. With the controlled different illumination parameters, the latency can cause the image sensor overexposed due to excessive illumination resulting from the delay of the bus. There is therefore a great need for an approach that results in a simple control of the lights and, more importantly, an adaptive timing control of the light integration process in the image sensor to produce balanced images in any conditions.
The present invention has been made in consideration of the above described problems and needs and has particular applications in imaging systems, such as image scanners, copy machines and facsimile machines, that optically convert an object to a signal or image before other processes can be proceeded.
An imaging system includes a sensing module that converts an object optically into an electronic image. Key components in the sensing module include an illumination source, an optical system and an image sensor. According to one aspect of the present invention, the illumination source comprises a high intensity light source that may be a fluorescent light or a light pipe excited by one or more light-emitting diodes (LEDs) and is typically turned on all the time for stabilization of the light for high-speed imaging applications. To avoid the image sensor being overexposed, a sensor control signal traditionally controlling the operation of the image sensor includes additional adjustable timing. parameters that ensure a light integration process in the image sensor is proceeded only for a predefined time regardless how long the image sensor is exposed to the illumination source. This guarantees that images generated are not subject to irregular imaging cycles due to various delays in the imaging system.
In the case of color imaging, the illumination source comprises at least three colored light groups, each preferably having one primary colored lights. According to the invention, a control signals circuitry provides illumination control signals with illumination parameters independent of the sensitivity of the image sensor, each controlling one of the colored light groups. The illumination-parameters may include predetermined exposure time intervals and powers. Each of the predetermined exposure time periods (intervals) controls the exposing time for which a colored light group applied by one of the powers is kept on. Those skilled in the art may appreciate that the use of illumination control signals with predetermined but non-controlled illumination parameters is a radical shift from the prior art systems using controlled illumination parameters to compensate for non-uniform spectral responses of the image sensor. Using predetermined but non-controlled illumination parameters significantly simplifies designs related to the illumination controls.
According to another aspect of the present invention, the present invention uses one sensor control signal to control the operation of the image sensor and particularly each of the light integration processes thereof so as to compensate for non-uniform spectral responses of the image sensor. This is achieved by using a sensor control signal comprising sequential sensing signals, each responsible for one light integration process under one colored light group. Further each of the sensing signals has a dynamically adjustable timing parameter adaptively to the spectral responses of the image sensor under a specified colored illumination so that the respective light integration process is always proceeded for a predefined time.
According to still another aspect of the present invention, the timing parameters for the sensing signals in the sensor control signal are determined or obtained by recursively comparing a set of test signals with a reference using adjusted timing parameters in the respective light integration processes. In this way, the commonly seen color biased problems in color imaging are minimized.
There are numerous benefits and advantages of having a sensor control signal to control the light integration process of a sensor and illumination control signals with predetermined pararmeters. Accordingly, one of the objects of the present invention is to provide an approach that results in a simple control of the illumination source and, more importantly, an adaptive timing control of the light integration process in the image sensor to produce balanced images in many irregular conditions.
Other benefits and advantages, together with the foregoing are attained in the exercise of the invention in the following description and resulting in the embodiment illustrated in the accompanying drawings.